The invention relates to a method for producing polypropylene fibers having high tenacity and low elongation, by continuously melt spinning, withdrawing, drawing, and cutting polypropylene fiber strands.
A single zone method of this type for producing high tenacity polypropylene fibers is known from DE 35 39 185.
In the known method, fiber strands advance after spinning into a water bath for the purposes of undergoing cooling. Subsequently, they undergo drawing in a draw zone at a high draw ratio of up to 10:1. As a result of cooling the fiber strands in the water bath relatively quickly, a relatively high partial orientation occurs in the surface region of the individual fibers with the consequence of a poor stretch capability. In addition, the known method is suitable only for producing relatively coarse fibers with an individual denier in a range greater than 3 dtex. The greater friction effect on the individual filaments alone, which is caused by the water bath, does not permit spinning fibers with finer deniers.
However, right along with the use of such fibers for reinforcing concrete, there is a desire for very high-tensile and fine fibers. DE 198 60 335 A1 discloses a polypropylene fiber, wherein the disadvantage of a rapid quench in a liquid was replaced with air cooling. While this measure permitted producing fiber strands with finer individual deniers, it had the disadvantage that the fibers exhibited a high elongation of more than 60%. However, it has been found in the reinforcement of concrete base bodies that the elongation values of the reinforcement fibers should approximately correspond to those of the base material that is to be reinforced, so that the fibers are able to absorb the load at the earliest possible time, and that the base body remains unaffected. Thus, fibers with relatively high elongation values are suited only to a limited extent.
EP 0 535 373 A1 discloses a polypropylene fiber, which has a required fine denier with a high tenacity, but which can be produced only in a costly, two-zone process. In this process, the fiber is temporarily stored after spinning and drawing, while receiving an additional treatment by impregnation. Subsequently, in a second stage, the fiber strands are again taken up, dried, and cut.
It is therefore an object of the invention to provide a method for producing high tenacity polypropylene fibers of the initially described type, which permits producing particularly fine fibers with an individual denier of less than 2 dtex.
A further object of the invention is to provide an apparatus for carrying out the method.